Separation of fine granular materials

ABSTRACT

This application concerns the separation of fine granular mixtures that may occur when the grains are suitably vibrated within fluid. In particular, the present invention provides a method of separating a particulate mixture comprising different particle types, comprising subjecting a supported particle bed comprising a fluid and said particulate mixture to a vibration thereby to effect separation of the particulate mixture into strata each of which is preferentially rich in substantially one particle type. Apparatus for effecting the above method also provided.

This application concerns the separation of fine granular mixtures thatmay occur when the grains are suitably vibrated within a fluid.

Separation of multi-particulate systems is an essential activity in themodern process industries. It is particularly important in mineralprocessing where high value materials are extracted in the presence offar lower value components such as silicates. Specialised methods forachieving this separation based on magnetic properties and electricalconductivity may be applicable in some cases. However, the majority ofseparation processes rely on differences in particle size, density, ormass, either through differential behaviour in air using cyclones orother inertial classifiers, or by behaviour in dense-phase systems suchas fluidised beds. “Wet” methods such as hydro-cyclones, sedimentationor froth flotation may also be used.

However, all of these processes fail to produce a complete separationwithin a narrow property range, certainly within a single process stage.At present, the emphasis is often either on concentrating the desiredcomponent of the mixture to a level where it is economical to performthe remainder of the extraction using chemical means, or on removing asmuch of the desired material as is economic using a multi-stage system.The extra processing may incur appreciable costs and yet still involveconsiderable waste. At present the effectiveness of separation,particularly in a single process, falls considerably for particle sizesbelow about 100 μm for processing in air and about 50 μm for wetprocessing.

Granular materials occur widely in nature and the ability to handlegrains and powders is central to numerous industrial processes (1, 2).The dynamics of large grains are influenced by the in-elastic nature ofcollisions (3); under vibration they may exhibit flow (4), convection(5, 6), arching (7, 8) and pattern formation (9, 10), while the “Brazilnut effect”, in which a larger grain moves to the top of a collection ofsmaller grains of similar density (11), is well known. For diametersless than about 0.5 mm air plays an important role, leading tospontaneous “Faraday” heaping and tilting under vertical vibration (12).

The present invention is based on extensive research which bringstogether these two important aspects of granular dynamics; the effect ofambient fluid and granular separation.

Since the time of Faraday, vibration-induced air-flow has been known toinfluence the motion of fine particulates (13), the simplestmanifestation being the spontaneous formation of heaps in verticallyvibrated granular layers (14). However, to date, there is no generalconsensus on the detailed mechanism responsible for this instability(15-17).

Similarly, there is now a substantial body of knowledge on segregationand separation in granular composites (18), but a clear understanding ofmany of the physical processes involved is still lacking. Furthermore,much of the attention has been focused on large particulate systems forwhich fluid effects are unimportant.

While particles of relatively large sizes can be segregated according todensity by flotation or cyclone methods, the separation of significantlysmaller particles can be extremely difficult by previously knownmethods. The method and apparatus of this invention can give a rapid andefficient separation of different particle types.

It would be desirable to provide an efficient single stage process thatis able to separate fine particulates. It would be further desirable toachieve high specificity in the separation of particulate materials. Itwould also be desirable to achieve separation at greater speeds thancurrently available.

It would further be desirable to achieve an efficient method ofseparation of particulate layers in a system whereby the particles havealready been separated into strata.

According to a first aspect of the present invention there is provided amethod of separating a particulate mixture comprising different particletypes, comprising subjecting a supported particle bed comprising a fluidand said particulate mixture to a vibration thereby to effect separationof the particulate mixture into strata each of which is preferentiallyrich in substantially one particle type.

In a particularly preferred embodiment, fluid is driven through theparticle bed concurrently with the application of the vibration. Thefluid is preferably driven in phase with the vibration. Typically, thefluid is driven through the bed by the oscillatory movement of a surfacesupporting or bounding the particulate bed. By “driven” we mean thatfluid is positively moved through the bed, not just that fluid ispresent.

In another particularly preferred embodiment, the particles from astrata formed by the method of the present invention may be extractedfrom the container by one or more extraction points. The extractionpoints may be disposed at different positions within the bed, theextraction preferably being effected by the application of a vibrationto the particle bed.

Preferably, the particle types differ in that each particle typecomprises particles having similar sizes and/or densities. Moreparticularly, as between the different particle types, values of theexpression d^(n)ρ preferably differ significantly, where d is the meandiameter, n is approximately 2 (±0.5), preferably 2, and ρ the densityof the particles of the respective different types. For example, d^(n)ρshould preferably differ by at least 5%. Thus, a particular particletype may comprise a distribution of particle sizes, but the d^(n)ρdistribution distinguishes different particle types. Strata are formedwherein a strata comprises particles of similar type, for example havinga similar d^(n)ρ value. Different strata are distinguished from oneanother in that they have substantially different particle types toother strata.

According to a further aspect, the present invention provides apparatusfor separating a particulate mixture comprising particles of differenttypes, wherein the apparatus comprises a support for the particulatemixture and a fluid medium, and a vibrator for subjecting the containerto vibration, thereby to effect separation of the particulate mixtureinto strata each of which is preferentially rich in substantially oneparticle type.

The fluid medium is suitably a gas, preferably air. Separation in a gasis usually termed “dry separation”. Separation may also be carried outunder some other gas, for example, a substantially inert gas such asnitrogen if desired. Liquids may also be effectively used as the fluid.In particular, water has been used to good effect. Separation using aliquid as fluid is termed “wet separation”. Preferably, the fluidsubstantially occupies the space between particles.

Preferably, the first and second types of particles are significantlydifferentially damped in their movement in a fluid. That is to say thatthe fluid in which the particles are vibrated damps the movement of oneparticle type to another particle type to significantly differentlevels. The effect of the damping force on a particle results from theinterplay of the viscous damping force acting on the particle and themass of the particle. A relatively large and heavy particle will be lessdamped in its motion than a relatively small and light particle.

Other embodiments of the invention provide that said fluid medium is asupercritical fluid. The use of a supercritical fluid allows theviscosity of the fluid medium to be altered from values typical of gasesto values more typical of liquids.

The present invention affords particular advantages in the separation ofparticles of different densities that have substantially the samegranulometry. This is thought to be of particular value in mineralseparation, especially in the extraction of the chemically less activemetals which occur in the metallic state. Ore containing such metals,e.g. platinum, iridium, may be finely crushed, possibly screened, andthen separated according to the invention. The high density metal willreadily separate from the lower density ground rock.

When we speak of the density of a particle or particle type, we denotethe actual density of that particle or the particles making up thattype, and not the bulk density of the type. When we speak of thediameter of a particle, we refer to the mean size of the particlesmaking up the particle type. For example, we can denote the mesh size ofthe smallest screen through which that particle will pass. We shall alsorefer to a G₅₀ grain size which is the mesh size of a screen throughwhich will pass 50% by mass of the particle type under consideration.Thus, particle types generally exist as phases of similar particles andsuch phases will be termed types hereinafter. Indeed, in a system with anumber of different particle sizes and/or densities, there may exist aplurality of strata or pseudo strata; in effect a rainbow of differentlayers comprising similar particle types. In excess of two particletypes may be resolved by the present invention. For example, 3, 4, 5 andmore particle types may be separated effectively and efficiently.

For dry separation, the invention has advantages in the separation ofparticles having a G₅₀ grain size of less than 500 μm, preferably lessthan 350 μm. Once the grain size of the particles reaches more than 500μm, the efficacy of the process drops and the tendency to separatediminishes.

At small particle sizes, for example, particles of less than 100 μm indiameter, the method of the present in invention has particularadvantages over other separation methods such as sieves. Sieves tend toblock at these lower particle sizes.

Operation of the invention for dry separation is especially effectivewhen said particles have a G₅₀ grain size between 10 μm and 250 μm, andthe advantages afforded by the invention are especially evident when theG₅₀ grain size of the particles is below 150 μm, most preferably below100 μm, since it is in that range of grain sizes that previously knownmethods of separation have been found wanting.

For wet separation, the invention has advantages in the separation ofparticles having a G₅₀ grain size of less than 3 mm, preferably lessthan 2 mm. Operation of the invention for wet separation is especiallyeffective when said particles have a G₅₀ grain size between 10 μm and 1mm.

It will be appreciated that due to the viscosity of liquids as comparedto gases, a liquid may be used to achieve different damping effects togases.

In a most preferred embodiment, as between the different particle types,values of the expression d^(n)ρ differ by at least 10%, preferably atleast 20%, more preferably 30%, where d is the mean diameter, n isapproximately 2, and ρ the density of the particles of the respectivedifferent types. Such particle types are readily separated by thepresent invention. Where values of the expression d^(n)ρ differ by morethan 30%, the mixtures become increasingly easy to separate.

There may be a plurality of different particle types present in themixture. Although the invention will work with a large number ofdifferent particle types, it is preferred to minimise the number ofdifferent types in order to enable a simpler and more efficientextraction of different types from the separated particulate mixture. Tothis end, the particulate mixtures used in method of the presentinvention may be subjected to an initial screening process to minimisethe number of particle types present. However, the process of thepresent invention is essentially a single stage process that providesclear advantages over the convoluted and multi-stage processes of theprior art.

The vibration applied to the particle bed is preferably in the range of3-500 Hz, more preferably 10-200 Hz, most preferably 30-80 Hz.

In one preferred embodiment, the vibration has a resolved amplitude inthe vertical or horizontal direction in the range of 90-100% of thevibrational amplitude, more preferably greater than 95% of thevibrational amplitude, most preferably greater than 98% of thevibrational amplitude.

Vibrations may be applied in conditions of reduced gravity, for examplezero gravity.

The type of vibrational waveform applied to the particle bed is ofimportance. Typical tests have been done with sinusoidal waveforms, butit has been observed that if non-sinusoidal vertical oscillations areused, unwanted effects may be controlled.

In some embodiments of the present invention, for example the partiallyfilled container discussed below, having separated the mixture intostrata, horizontal vibrations may be applied to cause the bed to acquirea particular configuration.

The vibration preferably substantially fluidizes the particulate bed.Preferably the vibration forces fluid through the bed during theapplication of vibration. It is postulated that separation is effectedas a result of viscous fluid force acting differentially on the variousparticles. Thus, types of particles that are sufficiently distinct intheir interaction with the fluid may be separated from one another.

The present invention can achieve above 90% separation in about a secondfor certain idealised particle types, for example relatively smooth,substantially spherical particles. Depending on the separationparameters, in the order of 98% separation can be achieved in the orderof 1-2 minutes, for less ideal particle mixtures, such as coal and sand.

In a preferred embodiment of the invention, Γ is preferably at least 2.Γ is the ratio of the maximum acceleration of the container to theacceleration due to gravity and is given by Γ=Aω₂/g where A is theamplitude of the oscillation, g is the standard acceleration due togravity and ω=2πf where f is the frequency of vibration.

The invention is most preferably operated so that the frequency andamplification of the vibration applied are controlled to give rise to asubstantially stable stratification of different types of theparticulate mixture. The control of vibrational parameters may beeffected by manual or automated means, for example, by computer program.Alternatively, or in addition, the waveform of the vertical vibrationapplied is controlled, and/or a horizontal component of vibration isapplied to stabilise stratification of separated particulate types. Thisalso simplifies selective extraction of separated types of particles.

If periodic, the form of the vibration may be sinusoidal or it may be ofa more general form. The latter may have advantages in one or more ofthe following: in increasing the effectiveness of separation; inreducing the number of separated bands; in conveniently positioning theseparated regions.

Relatively low frequency and high frequency vibration, as will bediscussed below, can lead to differing stratification of the particlemixture. For a system having two discrete types of particles, highfrequency may produce a band of one type of particle sandwiched betweenlayers of another type of particles. For low frequency vibration theremay just two discrete layers of different particulate types. Thisphenomena is discussed in more detail below.

The present invention works less effectively if the proportion of onetype of particles to the other type of particles is too low. In order toovercome this problem, carrier particles may be introduced. The carrierparticles are selected to have about the same size and/or density as thelow proportion of particles. More particularly, the carrier particlesare preferably selected to have about the same d^(n)ρ value as the typeof particles in the mixture which one wishes to extract. This adds“body” to the amount of particles that are present in only a lowconcentration. It is then possible to separate the particles moreefficiently, some of which are the target particles and others of whichare the “carrier” particles. Carrier particles are selected to be easilyseparated from the target particles. For example, by having magneticcarrier particles a magnet may be used to separate target from carrierparticles. However, any suitable method for separation of target andcarrier particles may be employed. For example, dissolution, flotationand the like. Additionally, a solid lubricant may be added to themixture to aid in the separation.

Particles may be separated in a batch process, or a continuous process.Indeed, the present invention finds particular utility in continuousprocessing as described below.

The separation apparatus comprise a number of components. The vibrationof the particle bed is preferably carried out on a support. The supportis preferably a container. The support may be made from any suitablematerial. Preferably, the particle bed is caused to vibrate by placingthem on a support or retaining them in a container and vibrating thesupport or container.

Where a container is used, it is preferably box shaped, and may besquare or rectangular. Alternatively, the container may be cylindrical.Preferably the base and walls of the container are substantiallyparallel or perpendicular to the axis of vibration. For example, wherethe vibration has a predominant resolved vertical component, the base ofthe container upon which the particle bed lies, is preferablysubstantially perpendicular to the vibration. Alternatively, if thevibration has a predominant resolved horizontal component, the base ofthe container upon which the particle bed lies, is substantiallyparallel to the vibration.

The dimensions of the container are not crucial and may be adjusted tothe type and scale of the material requiring separation. Preferably,where separation is carried out under substantially vertical vibration,the container is a box shape, having a greater height than width.Preferably, the container has a significantly higher height than width,for example, the aspect ratio of the container may be greater than 2:1,3:1 or 5:1. Conversely, where separation is carried out undersubstantially horizontal vibration, the container is a box shape, havinga greater width than height. Again aspect ratios of 2:1, 3:1 or 5:1 orgreater are envisaged.

The container preferably comprises a sensor for determining the amountof particulate matter in the container. This may be a weight or volumedetector. Alternatively, or in addition, the container may be providedwith a transparent section that allows an operator to determine thelevel of particulate mixture in the container and/or where differentstrata are located. Preferably, there are means provided for determiningflow rates of particulate matter into and/or out of the container.Sensors may also be provided to determine to what extent separation ofthe mixture is resolved.

A sensor may also be used to control the supply and/or extraction ofparticles to and/or from the container.

The vibration may be effected by any suitable means, preferably anelectromechanical vibrator, for example, a loudspeaker, a cam-bearingdriven shaft or a hydraulic vibrator. Preferably, the vibrator comprisesfrequency and/or amplification controls for controlling the vibrationalparameters of the vibrator. The controls may be manually operated orautomated.

The vibrator may be linked to a signal generator, the signal of whichmay be amplified before it is applied to the vibrator. The vibrator maybe controlled by a manual control input device, or an automated controldevice.

The control device preferably comprises means for controllingvibrational parameters, such as the frequency, resolution of vibrationin a particular direction, amplitude and the like. These may beinfluenced directly by an operator or may be controlled by a processorwhich analyses feed back information from sensors in the separationapparatus and controls the apparatus under the direction of a program.

It is observed that if a conduit, preferably a tube or pipe, is insertedinto the particulate bed, with the open end of the conduit at aparticular layer/region where particles have achieved separation, theseparated particles may flow along the conduit and out of the end—abovethe level of the top of the surface of the particle bed in the case ofvertical vibrations and a vertical conduit. This phenomena is maximisedwhen where the longitudinal axis of the conduit inserted into themixture is substantially parallel to the direction of vibration appliedto the mixture. For example, where the vibration has a predominantlyvertical resolved component of vibration, mixture will flow up a conduitplaced with its longitudinal axis inserted substantially vertically intothe mixture. Similarly, where the vibration has a predominantlyhorizontal resolved component of vibration, mixture will flow along aconduit placed with its longitudinal axis inserted substantiallyhorizontally into the mixture. Thus it is possible to have two conduitsinserted into a mixture, with their ends disposed at different materiallayers, so that one conduit conveys the separated first type ofparticles, and another conveys the separated second type of particles.The particle bed may be replenished with mixed material. By controllingthe diameters of the conduits and/or providing valve controls it ispossible to control the flow rates so that the container does notoverflow when a continuous process is achieved. This effect could betermed vibration induced particulate syphoning. The separated types maythen be conveyed for further processing.

Instead of, or in addition to, using this vibrational effect to extractmaterial from a layer a particulate type may be drawn off from saidvessel by more conventional means, for example, a screw feed mechanismor by aspiration.

While vibration at any angle from vertical to horizontal in envisaged,it is preferred that the particles are bounded by one or more walls thatare generally, preferably substantially perpendicular to the directionof vibration.

Where vibration-separation of particles of the same density is required,for example, small pharmaceutical crystals from large pharmaceuticalcrystals, vibration can readily separate particles with percentdifferences in size, for example, 30%, 50% or 100% is readily separable.

It is preferable to avoid the build up of static electricity which mayoccur in some circumstances. This may be achieved by vibrating for alimited period of time. Having a continual bleed of material away fromthe vibrating container is also preferable. This aids in dissipatingstatic electricity. The material of the container may be chosen tominimise static electricity. An anti-static aid may be added to theparticle-fluid system.

A surfactant may be added to a liquid system, for example water, toremove the tendency for the particles to flocculate. This tendency ismore pronounced at lower particle sizes. Alternatively, or in addition,surfactant may be added to the system as a wetting aid.

In a particularly preferred embodiment, the container is partiallyfilled. “Partially filled” means 90% of capacity or less. Preferably,the container is filled in the range of 25-75% of capacity. For apartially filled box, the vibration may, for an especially advantageousembodiment, be substantially vertically resolved. The vibration may besinusoidal and the spatial form of the separation depends upon bothangular frequency, ω=2πf, and the amplitude of vibration A, convenientlyexpressed as Γ=Aω²/g, where g is the acceleration due to gravity. Fordry separation there exists a low frequency form of separation in whichthe less fluid damped components are found above the more fluid dampedcomponents. There exists a high frequency form of separation in whichthe less fluid damped components are found as a sandwich between layersof the more fluid damped components. The regions for which thesedifferent behaviours are found may be determined by experiment. Thesephenomena are discussed in greater detail below. Optimal separation maybe obtained by following a particular path within the ω and Γ plane.There is also advantage in using a non-sinusoidal vibration, both interms of quality of separation and in terms of controlling the stabilityof the separation boundaries between the separated components. For wetseparation there exists a low frequency form of separation in which theless fluid damped component is found above the more fluid dampedcomponent.

The Faraday effect, which is well documented, is the way verticalvibrations of particles in a box create a sloping surface on the top ofthe particles. For a part filled container, this usually occurs in thelower frequency domain. At some low frequencies, oscillations may occurbetween the alternative tilts of the bed. There may be some applicationswhere such oscillatory tilting is not desired. To remedy this situation,it is possible to have a substantially vertical-only vibration at onetime, or for a first period of time (which may experience Faradaytilting of the bed surface), and to introduce a horizontal component ofvibration at a second time, or for a second period of time, in order tomanipulate the bed into a desired configuration. The application ofcontrolling vibration may be constant or interrupted, preferablyconstant. We have discovered that the introduction of a transversecomponent to the vibration enables us to control the movement of thesurface of the bed. In one embodiment, faster separation is achievedwith a substantially vertical only vibration and then the surface of thebed is controlled by introducing a horizontal component to the vibrationsubsequently.

In a further particularly preferred embodiment, the container may besubstantially filled with the mixture. This type of separation requiresthe container to be filled so that at the value of Γ applied, the grainsimpact both the upper and lower surfaces of the container. The morevigorously the vibrations are applied (higher Γ) the more accelerationfrom the vibrations predominate over gravity. However, the containermust not be filled to an extent which impairs fluidization. “Filled”means greater than 90% of capacity. For example, the container ispreferably filled in excess of the 92%, preferably 95%, most preferably98% of capacity. For a “filled” box, the vibration may be substantiallyvertically resolved. For sinusoidal vibration the spatial form of theseparation depends upon both angular frequency, ω=2πf, as defined above.The usual form of separation is then as a series of bands. These reducein number as the vibration is continued until a single sandwich occurs.This form of separation is found over a wide range of frequencies. Theseparation may occur as a single region of each class of grains if anon-sinusoidal vibration is applied.

In yet a further preferred embodiment, the direction of the vibrationmay be horizontal or close to horizontal. For this embodiment, thecontainer must then be “filled” as described above. For example, thecontainer is preferably filled in excess of the 92% capacity, morepreferably 95%, most preferably 98% capacity. This allows the granularmixture to be separated in such a way that the fluid is forced throughthe granular bed by the vibration. For this type of separation thevibration is applied horizontally and the container should be filled sothat, at the value of Γ used for the separation, the granular bed fillsthe container by fluidization, thus forcing the fluid through the bedduring each vibratory cycle. The usual form of separation is then as aseries of bands which reduce in number as the vibration is continueduntil a single sandwich occurs. This form of separation is found over awide range of frequencies.

The separation may occur as a single region of each class of grains if anon-sinusoidal vibration is applied.

In order to illustrate the invention, certain processes will now bedescribed by way of example only and with reference to the accompanyingdrawings in which:

FIG. 1 shows a view of the experimental apparatus showing a glass boxand box mount (a), an accelerometer (b), two electromagnetic transducers(c) and a connecting frame (d).

FIG. 2 shows the behaviour of mixture A1, as a function of frequency andΓ, showing the onset of “bronze on top” (α), the onset of sandwichseparation (β), a transition boundary between the two (γ), and the onsetof slow (δ) and rapid (ε) inversion oscillations of a first kind. Alsoshown are the regions of “bronze on top” (C), violent thrashing andthrowing (A), simple tilt oscillations (B), sandwich configuration (D),oscillations between “bronze on top” and the sandwich configuration (E),continuous inversion oscillations of the first kind (F), and continuousinversion oscillations of a second kind (G).

FIG. 3 shows the behaviour of mixture A1 under Γ=6.8 at 160 Hz showingthe formation of the sandwich configuration. The pictures form a timesequence from upper left to lower right. The fifth picture was takenafter 50 s and the ninth picture after 7 mins. The bronze-rich regionappears dark-gray, while the glass appears white.

FIG. 4 shows the behaviour of mixture A1 under Γ=16.7 at 70 Hz showingone half period of a continuous inversion oscillation of the first kind.The pictures form a time sequence from upper left to lower right. Thetime period of full oscillation is 30 s.

FIG. 5 shows the behaviour of mixture A1 under Γ=16 at 70 Hz showingpart of a continuous inversion oscillation of the second kind. Thepictures form a time sequence from upper left to lower right. The timeperiod of full oscillation is 37 s. Note the formation of a number ofsmall bronze fragments, which stay intact, later joining the main bodyof bronze.

FIG. 6 shows the behaviour of mixture A2, as a function of frequency andΓ, showing the onset of “bronze on top” (α), the onset of sandwichseparation (β) and the transition between the two (γ and δ). Also shownare the regions of “bronze on top” (C), violent thrashing and throwing(A), simple tilt oscillations (B), sandwich formation (D) andoscillations between “bronze on top” and tilt (E). The pseudo-sandwichzone between the lines δ and γ is indicted as F.

FIG. 7 shows the behaviour of mixture A3, as a function of frequency andΓ, showing the onset of “bronze on top” (α), the onset ofpseudo-sandwich separation (β) and the transition boundary between thetwo (γ). Also shown are the regions of “bronze on top” (C), violentthrashing and throwing (A), simple tilt oscillations (B), and thepseudo-sandwich configuration (D). Oscillations between “bronze on top”and the pseudo-sandwich configuration occur at E.

FIG. 8 shows the behaviour of mixture B1, as a function of frequency andΓ, showing the onset of “bronze on top” (α), the onset of sandwichseparation (β) the transition boundary between the two (γ), and theonset of slow (δ) and rapid (ε) inversion oscillations of the firstkind. Also shown are the regions of “bronze on top” (C), violentthrashing and throwing (A), simple tilt oscillations (B), the sandwichconfiguration (D), oscillations between “bronze on top” and the sandwichconfiguration (E) and continuous inversion oscillations of the firstfind (F). In the zone G phenomena similar to continuous inversionoscillations of the second kind occur very sporadically.

FIGS. 9 to 12 show various examples of a particle separation apparatus.

EXAMPLES

The experiments described use bronze spheres of density ρ_(b)=8900 kg/m³and soda-glass spheres of density ρ_(g)=2500 kg/m³. The dynamic anglesof repose lie within 23.4°±0.8° and 23.9°±0.8° for the glass and bronzespheres respectively. The elastic properties have been studied byvibrating glass and bronze spheres in a glass box under vacuum, atvelocities comparable to those in the present experiments. Thecoefficients of restitution appear to be comparable and close to unity.

Spheres of closely similar sizes often form close packed crystallinestructures under vibration, with bulk movement of a whole crystallineblock rather than independent movement of individual grains. The glassand bronze spheres are therefore sieved to produce a size-spread inorder to avoid crystallisation effects.

We have studied four ratios of the mean diameters. Mixtures A consist ofbronze spheres with diameters in the range 125-150 μm and glass sphereswith diameters in the range 63-90 μm. Mixtures B consist of bronze inthe range 90-125 μm and glass in the range 90-125 μm. Mixtures Cconsists of bronze in the range 63-90 μm and glass in the range 125-150μm, while mixtures D consist of bronze in the range 45-53 μm and glassin the range 125-150 μm. For each class of mixture we have studiedvolume ratios of the bronze to glass components of 25%:75% (referred toas 1), a 50%:50% mix (referred to as 2) and a 75%:25% mix (referred toas 3).

A chosen mixture, of mean depth 20 mm, is contained within a rectangularsoda-glass box 50 mm high. Internal dimensions of 40 mm times 10 mm inthe horizontal plane have been used unless otherwise noted. The boxesare vibrated using the arrangement shown in FIG. 1. The glass box isglued to a metal mount which is bolted to a rigid frame held between apair of electromagnetic transducers. The transducer assembly is attachedto a massive concrete block (approx 250 kg). This configuration ensuresaccurate one dimensional motion over the frequency range of interest (10Hz<f<200 Hz), the axis of vibration being aligned to the vertical towithin 0.2°. The vertical motion is monitored with a pair of capacitancecantilever accelerometers, covering the ranges 0-8 g, and 0-80 g. Thewaveform of the vibration is monitored on an oscilloscope.

Persistently shaken fine glass spheres develop static charge. They maythen stick to the walls, impairing photography. If very vigorous shakingis continued for some time, the build up of charge eventually begins toaffect the granular dynamics, slowing the progress of many of thephenomena which we will describe. The addition of minute quantities ofan anti-static surfactant slows the build up of static withoutappreciably influencing the dynamics. However, in the present studies,we have preferred to replace the mixture being studied with a freshlyprepared one, should appreciable static effects become evident.

Varying the Composition

The effect of varying the bronze/glass composition is studied bycomparing and contrasting the behaviour of mixtures A1, A2 and A3, wherethe bronze and glass diameters are 125-150 μm and 63-90 μm and thebronze percentages by volume are 25%, 50% and 75% respectively. Anoutline of the behaviours observed for mixture A1 as a function of f andΓ is shown schematically in FIG. 2. The data points have been obtainedby slowly increasing Γ at a number of fixed frequencies, starting from awell mixed state.

At lower frequencies global convection and tilting of the upper surfaceare observed as the amplitude of vibration is increased. At the lineshown in FIG. 1 as α, sharp separation boundaries quickly appear betweenglass-rich and bronze-rich regions. The bronze-rich regions rapidlymerge into a single upper bronze-rich “phase” which lies above a lowerglass-rich “phase”, the “bronze on top” configuration. The boundarybetween phases is extremely well defined, being only one grain-diameterwide. Development towards a single upper bronze-rich region occurs bycoarsening, an effect observed in other granular systems. Eventually,the bronze-rich phase contains a small proportion of glass, estimated byvisual inspection, following the cessation of vibration, to lie in therange 2-20% depending upon f and Γ, while the lower region consistsalmost entirely of glass.

In the “bronze on top” configuration, convection currents occur withinthe individual bronze and glass-rich regions but they do not act tocause mixing. Above α, in the “bronze on top” regions A and C both theseparation boundary and the upper surface exhibit wave-likefluctuations. At low values of Γ and f, in the region B in FIG. 2,simple oscillations back and forth between the two alternative tiltsoccur. At higher values of Γ the fluctuations contain both periodic andnon-periodic components. For values of Γ corresponding to region A,considerable “writhing” of the interface between the glass andbronze-rich regions is observed together with writhing and throwing ofthe upper surface. Surprisingly, a sharp separation boundary betweendistinct phases is maintained despite these disturbances. However, inthe higher parts of region A the writhing and throwing becomes so severethat glass breaks through the upper bronze layer. Partial separation isstill maintained, with a sharp separation boundary, but the bronze-richregion now contains a higher proportion of glass.

At higher frequencies there is also a line at which sharp separationboundaries between a bronze-rich phase and a glass-rich phase appear, βin FIG. 2. Here, however, the bronze-rich regions rapidly merge to forma stable single layer at an intermediate height, between upper and lowerglass-rich regions. This we refer to as the sandwich configuration. Theupper surface of a sandwich is close to horizontal. Again, thebronze-rich layer contains a proportion of glass but the glass-richregions are almost completely free of bronze. The formation of asandwich configuration is illustrated in FIG. 3. Bronze concentrates inthe region of what will become the “sandwich filling”. In the earlystages this region also contains pockets rich in glass but they are thenejected as the sandwich develops. In the sandwich region the glass phaseis eventually almost pure glass while the bronze-rich phase contains4-20% of glass depending upon f and Γ.

In the sandwich regime, D, and for some higher values of Γ, theformation of glass “rain-drops” may be observed within the bronze-richlayer. These drops then either fall into the lower glass layer or riseto join the upper glass layer. It appears that some glass is continuallybut slowly passing into the bronze-rich layer and that this process ofdroplets rising or falling maintains equilibrium.

In the region E, where lines α and β meet, the formation of glass-richand bronze-rich regions involves complex oscillations between the“bronze on top” configuration of region C and the sandwich configurationof region D.

The position of the lines α and β depends somewhat upon the time scaleof measurement. α and β have been chosen to correspond to the formationof sharp separation boundaries and the substantial completion ofseparation into distinct glass-rich and bronze-rich phases after about 2minutes. If Γ is lowered by 10% the formation takes very many minutes,while at 10% higher the, separation is substantially complete in muchless than one minute.

Following separation, convection may be observed within the individualglass and bronze-rich regions, with considerable velocity shear at thesharp separation boundaries but no convection currents are present whichwould mix the two phases. In region C tilted configurations are oftenfound where this shear is particularly evident, since the bronze-richand glass-rich regions then have convection currents in the same sense.Both at low and at high frequencies, we observe considerably morekinetic activity in the bronze-rich phase than in the glass-rich phasewith correspondingly greater speeds of convection in the former.

The boundary between the “bronze on top” region, C, and the sandwichregion, D, is shown as the line γ. As this line is approached from belowor from the left the upper bronze layer avalanches down the tilted slopeand passes into the depths as a cylindrical roll, then spreading out toform a sandwich between upper and lower glass-rich layers. We refer tothis process as an inversion. At no stage are the sharp separationboundaries between the two phases lost. If the line γ is approached fromthe right the sandwich configuration slowly changes to the “bronze ontop” configuration, unaided by an inversion. Rather the configurationtransforms continuously by the glass diffusing from the upper glasslayer through the bronze-rich region into the lower glass layer. Atlower values of Γ this can take a very long time indeed.

In the region indicated as F the inversion process repeats continuously.One half period of such a process is shown in FIG. 4. Upon entering Ffrom the region C the bronze-rich layer of the sandwich rises slowly tothe surface, while remaining close to horizontal. Once there, thesurface tilts. The bronze then avalanches down the slope, and passesinto the depths of the glass to form a stable low-lying horizontallayer. The bronze then slowly rises to the surface again and the processis repeated. We describe this behaviour as a continuous inversion of thefirst kind. At the line marked δ the period of this repeating process ismany minutes, while by the line ε the period has fallen to about oneminute.

Within the region G an alternative type of oscillation is found,continuous inversions of the second kind. Such a process is shown inFIG. 5. The bronze layer moves towards the upper surface of the bed, butbefore it reaches the upper surface it necks and divides in the middle.Glass thrusts upwards through the gap, the two bronze regions passingdown the right and left sides of the box to reform a single sandwichlayer low in the bed. During this process the main bodies of bronze sheda number of small fragments. Each of these sharp boundaried fragmentsremains intact, eventually joining the single sandwich layer. Thissingle horizontal layer then slowly moves to the surface and the processrepeats. The onset of this oscillation is somewhat erratic and we finddifficulty in determining the boundaries to this form of behaviour.

An outline of the behaviour of mixture A2 as a function of f and Γ isshown schematically in FIG. 6. Again, the data shown have been obtainedby slowly increasing Γ at fixed frequencies, starting from a well mixedstate. As with mixture A1, at low frequencies we observe the onset ofseparation into a “bronze on top” configuration, at the line marked α Athigher frequencies we observe separation into the sandwichconfiguration, the onset occurring at the line β. At the intersection ofthe α and β lines there is a small region marked E at which complicatedoscillations between the “bronze on top” and the sandwich configurationsoccur. We note in FIG. 6 the region A in which active writhing of theseparation interface and violent throwing of the upper surface occurs.However, for this mixture composition the bronze-rich region remainsuppermost, despite the activity. Simple tilt oscillations occur inregion B. The boundary between the extensive “bronze on top” region, C,and the sandwich region, D, is also shown. As Γ is slowly increased atintermediate frequencies, above the β line, the top layer of thesandwich thins by the diffusion of glass through the central bronzelayer. Eventually the upper layer fails to cover the bronze layercompletely, by the line γ. We refer to this configuration as a“pseudo-sandwich”. For sufficiently high Γ the upper glass withdrawscompletely and the “bronze-on top” configuration is found, at the lineδ. These processes are very slow and very many minutes must be allowedfor each measurement. If the upper surface is symmetrically domed due totwin convection cells, an arrangement often found at higher frequencies,the pseudo-sandwich will have glass both at the right and left-handextremes of the box. Pseudo-sandwiches also occur when the upper surfaceis tilted. Glass may then be found only at either the upper or the lowerregions of the slope. For mixture A2 the tilted configuration is foundclose to region E while at higher frequencies the symmetricalconfiguration is preferred.

For mixture A2 we find no inversion oscillations. If Γ is kept constantwhile f is varied to take the system between region C and D the processof transformation between the “bronze on top” and sandwichconfigurations is very slow and the behaviour is hysteretic even whenobserved on time scales of minutes.

The equivalent information for mixture A3 is shown in FIG. 7. Once againwe observe the formation of the “bronze on top” configuration at theline α. No failure of separation occurs in region A. For A3 we observeno full sandwich formation, rather the onset of a region of symmetricpseudo-sandwich formation at the line β. Complicated oscillationsinvolving the two configurations occur at the intermediary region E. AsΓ is increased above the line β, the upper glass regions of thepseudo-sandwich retreat, the full “bronze on top” configuration beingrecovered by the line γ. We observe no inversion processes anywhere inthe f-Γ plane, transformations of configuration occurring by the slowdiffusion of glass through the bronze-rich layer.

In all of the experiments within the 10 mm×40 mm box the principleseparation features are close to two dimensional, the configuration ofthe bronze-rich and glass-rich regions being very similar when viewedthrough the opposite large faces of the box.

In comparing the behaviours of A1, A2 and A3, we note the following. Allmixtures exhibit excellent separation with extremely sharp separationboundaries. The onset of separation with increasing Γ occurs at lines aand β which rise only slightly in Γ as the bronze proportion isincreased. The interface region between α and β occurs at systematicallylower frequencies as the bronze proportion is increased. At the sametime the region of “bronze on top” behaviour increases, the region ofsandwich behaviour found in A1 withdrawing to a restricted area ofpseudo-sandwich behaviour by A3.

We note that inversion processes only occur for bronze-poor mixtures,such as A1. More usually, a new configuration of the bronze-rich andglass-rich regions occurs by the slow process of diffusion. If similarexperiments are repeated in boxes having a squarer cross-section verysimilar processes of separation into “bronze on top”, sandwich orpseudo-sandwich configuration occur. However, inversion operations areeffectively suppressed by the use of a 10 mm×10 mm box. Restricting thebox dimension in this way also suppresses tilt oscillations.

The data points in FIGS. 2, 6 and 7 are each for a particular sample ofthat composition. The onset lines α and β do not vary greatly withcomposition, while the upper structures of FIGS. 2, 6 and 7 changegreatly with composition. This behaviour is reflected in the variationof our data between samples of the same nominal composition. While theonset lines α and β are reproducible from sample to sample to withinabout ±5% in Γ, there is considerably more variation of the upper lines.The boundaries of the region F of FIG. 2 for example show largervariations of behaviour from sample to sample. The lower limit of theregion F of freshly prepared samples of mixture A1 measured on a timescale of some minutes varies from Γ=8 to about Γ=11. This almostcertainly reflects variations in composition. While we have sieved thesamples to have sizes within certain limits, we have not controlled thedistribution of sizes between these limits.

Variation of Size Ratio

If differential air damping is the dominant separation mechanism, onewould expect separation for some size ratios of the two components andnot for others. For a mixture of bronze and glass particles of densitiesρ_(b) and ρ_(g) and mean diameters d_(b) and d_(g) respectively, asimple dynamical model based on Stokes' law suggests that the relativestrength of the air-effects on the two components may be characterisedby the ratio S=(ρ_(b)d² _(b))/(ρ_(g)d² _(g)).

We now compare the behaviours of four types of mixture having differentvalues of this parameter S.

For the mixtures A1, A2 and A3 which we have just discussed S=12, theglass component being far more heavily damped. The separation displayssharp boundaries with clear regions of “bronze on top”, sandwich andpseudo-sandwich behaviour.

The mixtures B have S=3.6, the glass component being appreciably moredamped by the air. Preliminary data for mixture of composition B1 hasalready been reported by Burtally et al [19]. FIG. 8 shows the resultsof a more detailed study using the same time-scale criteria as thoseused above.

There are many similarities to the data for A1. The separation lines, α,for “bronze on top” and β for sandwich formation again appear, as doesthe line γ at which transformation between the two configurations occursby inversion when approaching from below or from the left. Failure ofseparation may occur in the upper parts of region A. The region F is oneof continuous inversions of the first kind. However, the region ofcontinuous inversion oscillations of the second kind found at very highf and Γ for A1 is even harder to define for B1, where the onset of veryragged and far from symmetrical inversions happens on a very erratic anddelayed basis within the region G.

The behaviour of mixture A2 has many similarities to that of B2.Sandwich behaviour is now found over a limited range of Γ at higherfrequencies, the transformation into “bronze on top” as Γ is raisedoccurring by diffusion, passing through a pseudo-sandwich configuration.In B3 only pseudo-sandwich behaviour is observed, over much the sameregions of f and Γ as for A3.

The principal differences between the behaviours of the A and B seriesof mixtures are: the bronze-rich regions of the B mixtures contain lessglass than the A mixtures under corresponding conditions; the formationof glass droplets within the bronze-rich phase and resulting rainfall isfar less evident for mixtures A than in the case of B; the dynamics ofthe A series are somewhat faster than for the B series over much of thef-Γ plane.

For the mixtures C the parameter S=1. For C1 we observe very poorseparation, with traces of sharp boundaries only visible over thelimited frequency range of 25-90 Hz at lower values of Γ. We observe theformation of bronze-enhanced regions close to the upper surface at somelower frequencies and bronze-enhanced regions at intermediate levels atsome higher frequencies. We also observe some oscillatory behaviours ofthe bronze-enhanced regions. However, the separation is always verypoor; the sharp separation boundary, where it does exist, distinguishesregions containing a considerable proportion of the other component. Atall frequencies increasing Γ readily induces global convection currentswhich thwart any tendency to separate. Mixtures C2 and C3 exhibit evenweaker tendencies to separate.

For the mixtures D, S=0.4. The air-damping of the bronze is now greaterthan that of the glass. For mixture D1, separation with glass uppermostis experienced at all frequencies investigated as Γ is slowly increased.Sharp separation boundaries are observed. The onset of separation occursat about Γ=2 for frequencies about 100 Hz, reducing to Γ=1.6 at 20 Hzand Γ=1.7 at 180 Hz. At lower frequencies, almost pure glass forms abovea bronze-rich lower layer. At higher frequencies the lower bronze regionis domed, due to the influence of the convection cells in the twomaterials. As Γ is increased, at any particular frequency, globalconvection causes mixing. This occurs at Γ=3.2 around 100 Hz, and atΓ=2.3 both at 20 Hz and 180 Hz. For higher values of Γ weak attempts atseparation are thwarted by rapid global convection. Nevertheless,attempts at sandwich formation can be identified at higher frequencies.

As the bronze concentration is increased the band of Γ values for which“glass on top” separation is found narrows and for D3 the sharpness ofthe boundary between the glass-rich and the bronze-rich regions is morediffuse.

In summary, we observe a very strong separation mechanism for mixturesA, particularly for those richer in glass. Small bronze-rich regionsremain intact with sharp boundaries even when separated from the mainbronze body under vigorous dynamics. Mixtures B exhibit an only slightlyweaker tendency to separate. For mixtures A and B the dominant tendencyis for the bronze to separate to the top. For mixture C, where the twospecies are estimated to be equally damped, only a very weak tendency toseparate is exhibited, while for mixtures D there is a tendency toseparate with glass at the top. This separation is, however, appreciablyweaker than for A and B and is thwarted at higher Γ by globalconvection.

The crude basis for the use of the parameter S should be noted. It isclear that S is not the sole defining parameter; the size ratio willclearly play some role, especially for small values of S, and sizesegregation mechanisms may come into play. Nevertheless, these resultsprovide broad support for the differential air-damping hypothesis.

Further Experiments

Burtally et al [19] have reported that the separation of mixture B1disappears at sufficiently low pressures which vary approximatelylinearly with frequency. These pressures are in reasonable agreementwith a simple theory based on treating the system as a granular bed andusing Stokes' law to introduce viscous damping. We have repeated theseexperiments on mixtures A and B and find similar pressures for the twomixtures. Vibration is applied to cause separation and the pressure isthen slowly lowered. Through a range of sufficiently low pressures theseparation boundaries become more diffuse. The glass-rich regionincreasingly contains more bronze while the bronze-rich region hasincreasing glass concentration. The rate of convection increases, withappreciable downward convection at the larger faces of the box.Convective currents eventually cause global mixing. For A1 this occursat pressures of about 20-25 mbar at 40 Hz and 75-85 mbar at 160 Hz.These figures are not appreciably different from those for mixture B1.We have further tested the involvement of air in the separation processby conducting experiments using a box constructed with glass walls butwith a porous bottom. The bottom surface consists of a layer of 63 μmwoven steel sieve mesh supported by a 3 mm layer of metal foam. Thisstructure is extremely porous to air, while being rigid to particlecollisions. The top of the box may be open or closed, using small bungs.

With the top of the box open we find no tendency to separate in any ofour mixtures. Rather we observe global mixing convection. With the topof the box closed, however, we observe the separation of mixtures A andB with a bronze-rich layer uppermost. This causes us to believe that itis necessary to positively drive the fluid through the bed of particles,not simply to have the fluid present.

Discussion

We have produced evidence for very strong separation effects both inglass/bronze mixtures of equal size but also in other mixtures,particularly those where the parameter S is considerably greater thanunity. These separation effects are noteworthy, both for the very sharpinterfaces between homogeneous bronze-rich and glass-rich phases, andfor the immunity of these phases and interfaces to violent disturbances.We have shown, for example, that both large and small fragments of thebronze-rich phase may circulate during inversion oscillations, with theinterface intact. A curious but key feature of the separation is theoccurrence solely of convection within each component, rather thanglobal mixing convection.

For each of the mixtures A and B we can produce separation withinseconds, in which the glass is essentially pure and the bronze containsonly 1-2% of glass by volume. It is clear that the separation mechanismis based on the interstitial air. All separation effects vanish atsufficiently low pressures. The magnitudes and frequency dependence ofthe failure pressure also supports this hypothesis. We note that as theparticle sizes are both increased, the separation mechanism weakens, thesharp separation boundaries becoming increasingly diffuse. Theseparation effects have largely disappeared by sizes of 400 μm. Thisfailure for large particles is to be expected for an air-drivenmechanism. The supposition that it is the differential air damping ofthe two species is supported by the present experiments in which theparameter S is varied and by the visual observation of the far greaterkinetic activity in the bronze rich phases in mixtures A and B. In the Dmixtures there is greater activity in the upper glass phase.

Given a strong tendency for the glass and bronze to separate, based ondifferential air damping, many of the features we observe can be readilyexplained. The more damped phase, the glass-rich phase in the case ofmixtures A and B, will be relatively compact and inert. It will bedifficult for bronze to re-enter such a phase. The bronze-rich phase onthe other hand will be relatively dilate; glass will find it possible toenter. Equilibrium will be maintained by the formation of glass dropletswhich subsequently “rain” upwards or downwards. In mixtures A the bronzeis larger than in mixtures B; there will be more interstitial space inthe bronze rich phase. This would explain why we observe a greaterproportion of glass within the bronze-rich phase of A than within thatof B with correspondingly more droplet formation and rainfall in A.

Other aspects are harder to deduce from the mere existence of a strongdifferential air damping separation mechanism. The preference for the“bronze on top” or the sandwich configuration, the position of thesandwich layer in the later case, and the inversion behaviours are moredifficult to predict. The curious relationship between local rather thanglobal convection and the separation itself also need clarification.

Burtally et al [19] have noted that regions slightly richer in the lessdamped component will be more dilate. The more active component willpreferentially tend to diffuse to such a region enhancing thatcomponent. It is suggested that this may cause a dynamical instabilityleading to “phase separation”. Our present experiments using a porousbottomed box have shown that just the presence of air is not sufficientto cause separation. If air is free to move up and down with thegranular bed, no separation occurs. However, in a solid bottomed box,air is forced through the bed as it moves with respect to the box duringthe vibration. Strong separation may then occur. The case of the porousbottomed box with a sealed top is intermediate between these twosituations since, due to the volume of air above the granular bed, airis only partially forced through the bed during vibration. “Bronze ontop”, but not sandwich formation, is then observed. It is clear fromthis key piece of evidence that the separation mechanism is more subtlethan that proposed by Burtally et al [19]. The situation may beclarified by suitable computer simulations. Preliminary simulations,using Stokes' law to incorporate viscous damping, confirm thatseparation is dependent upon air being forced through the bed.

FIG. 9 shows a container 100, having a continuous supply of a mixture offirst and second particles via inlet pipe 102. It should be understoodthat this arrangement may be used for both batch or continuousprocesses. This example shows a partially filled box. The box isvibrated vertically by an electromechanical oscillator 104. Within amatter of seconds two distinct bands form, with a sharp boundary betweenthem. There is an upper band of denser material 106 and a lowerband/layer of less dense material 108. This presupposes that theparticles going in are of similar diameter, but different densities.

A first extraction conduit 112 extends into the upper layer 106, and asecond extraction conduit 110 extends into the lower layer 108. We havefound that if tubes are placed into the layers as shown in FIG. 9 thereis an outflow of separated particles from the layer. This provides a wayof continuously extracting particles from the layers, after they havebeen separated into the different kinds of particle. Thus a continuousprocess may be effected. The flow rates should be manipulated somaterial may be added via pipe 102 and extracted by pipes 110 and 112.It may be desirable to have the position of the lower end of the pipes110 and 112 adjustable so as to ensure that they are properly located inthe appropriate layer of particles.

The electromechanical oscillator is controlled by a manual control inputdevice 113. The manual control input is linked to a signal generator 114and an amplifier 115. The amplifier is linked to the electromechanicaloscillator.

Of course, outlets for the particles may be provided at the side of thebox, rather than up vertical pipes.

FIG. 10 shows an partially filled container arrangement similar to FIG.9, but with the frequency of vibration this time being at a relativelyhigh frequency—so that three layers are formed rather than the twolayers that were formed shown in FIG. 9 (relatively low frequency). Themiddle layer is a denser layer than the top and bottom layers. Pipe 110is seen to delve into the lower less dense layer, while pipe 112 delvesinto the denser “sandwiched” layer.

FIG. 11 shows a “filled” container arrangement to which verticalvibrations are applied. Pipes 110, and 112 are shown to delve into theseparated particulate bed. A small amount of space is left in thecontainer 116 in order that dilation of the bed may be achieved duringvibration.

FIG. 12 shows an arrangement of a “filled” container that has beensubjected to substantially horizontal vibration at high frequency. Thisleads to a sandwich layer formation as in FIGS. 10 and 11, except thatthe layers are resolved into substantially vertical strata. A smallamount of space is left in the container 116 in order that dilation ofthe bed may be achieved during vibration. Pipes 110, 111 and 112 areshown to delve into the separated particulate bed.

We have performed tests with non-ideal particles, i.e., particles nothaving a uniform shape, for example not being substantially spherical.In one example, we have performed tests with sand and coal dustparticles. It has been found that the methods of the present inventionwork extremely well on these mixtures. The less regular nature of theparticles shape does not appear to adversely affect the principles northe efficacy of the present invention.

REFERENCES

-   1. H. M. Jaeger, S. R. Nagel, R. P. Behringer, Rev. Mod. Phys. 68,    1259 (1996).-   2. H. J. Herrmann, J.-P. Hovi, S. Luding, Eds., Physics of Dry    Granular Media (NATO ASI Series E, Vol. 350, Kluwer, Dordrecht,    (1998).-   3. S. McNamara, W. R. Young, Phys. Rev. E 53, 5089 (1996).-   4. J. Rajchenbach, Adv. Phys. 49, 229 (2000).-   5. J. B. Knight et al., Phys. Rev. E 54, 5726 (1996).-   6. K. M. Aoki, T. Akiyama, Y. Maki, T. Watanabe, Phys. Rev. E 54,    874 (1996).-   7. C. R. Wasgren, C. E. Brennen, M. L. Hunt, J. Appl. Mechanics 63,    712 (1996).-   8. S. S. Hsiau, S. J. Pan, Powder Technology 96, 219 (1998).-   9. C. Bizon, M. D. Shattuck, J. B. Swift, W. D. McCormick, H. L.    Swinney, Phys. Rev. Lett. 80, 57 (1998).-   10. P. K. Das, D. Blair, Phys. Lett. A 242, 326 (1998).-   11. A. Rosato, K. J. Strandburg, F. Prinz, R. H. Swendsen, Phys.    Rev. Lett. 58, 1038 (1987).-   12. H. K. Pak, E. Van Doorn, R. P. Behringer, Phys. Rev. Lett. 74,    4643 (1995).-   13. M. Faraday, Philos. Trans. R. Soc. London 52, 299 (1831).-   14. P. Evesque, J. Rajchenbach, Phys. Rev. Lett. 62, 44 (1989).-   15. J. Duran, Phys. Rev. Lett. 84, 5126 (2000).-   16. B. Thomas, A. M. Squires, Phys. Rev. Lett. 81, 574 (1998).-   17. K. Kumar, E. Falcon, K. M. S. Bajaj, S. Fauve, Physica A 270, 97    (1999).-   18. J. M. Ottino, D. V. Khakhar, Annu. Rev. Fluid Mech. 32, 55    (2000).-   19. N. Burtally, P. J. King, M. R. Swift, Science, 295, 1877-1879,    (2002).

1. A method of separating a particulate mixture comprising differentparticle types comprising: supporting the particulate mixture by a fluidimpermeable support; and vibrating the fluid impermeable support todrive fluid, interstitial to particles of the particulate mixture,through the particulate mixture at an amplitude and frequency to producedamping forces on substantially all the particles wherein a differencebetween the damping forces on each particle type causes the particulatemixture to separate into strata, each stratum being rich in at least oneparticle type.
 2. The method of claim 1, comprising selecting the fluidsuch that the fluid has the required viscosity to achieve the requireddamping forces to cause the separation of the different particle typesduring vibration of the fluid impermeable support.
 3. The method ofclaim 1 comprising defining a range of frequencies of vibration of thefluid impermeable support, over which separation of the particulatemixture into the strata is effected, for a given particulate mixture,and vibrating the fluid impermeable support at a frequency within saidrange of frequencies.
 4. The method of claim 3 comprising varying thedegree of separation of the particulate mixture within said range offrequencies.
 5. The method of claim 1 comprising providing the fluid inthe form of a gas.
 6. The method of claim 5 comprising providing the gasin the form of air.
 7. The method of claim 1 comprising providing thefluid in the form of a liquid.
 8. The method of claim 7 comprisingproviding the liquid in the form of water.
 9. The method of claim 1performed on a mixture having particle types having a G₅₀ grain size ofbetween 20 and 500 μm.
 10. The method of claim 1 performed on aparticulate mixture having at least two component particle types whichhave values of the expression d^(n)ρ that differ by at least 20%,wherein d is the mean diameter of particles of the respective particletypes, n is 2 and ρ is the density of particles of the respectiveparticle types.
 11. The method of claim 10 comprising separating theparticulate mixture such that a stratum that is substantially comprisedof at least one of a more dense particle type and a larger particle typeoverlies a stratum substantially comprised of at least one of a lessdense particle type and a smaller particle type.
 12. The method of claim1 comprising extracting particles from at least one stratum and at leastone extraction point.
 13. A method of separating a particulate mixturecomprising different particle types, the method comprising: supportingthe particulate mixture by a fluid impermeable support; driving fluid,interstitial to particles of the particulate mixture, through theparticulate mixture by vibrating the fluid impermeable support in orderto separate the particulate mixture into strata, each stratum being richin at least one particle type; and selecting a frequency and anamplitude of the vibration of the fluid impermeable support to have avalue of gamma between 2 and 18 and a frequency between 0 Hertz and 200Hertz.